1. Field of the Invention
The present invention relates to a circularly polarized wave antenna, and particularly, to a circularly polarized wave antenna excited in a higher order mode such as in a DAB (Digital Audio Broadcast) system, and to a manufacturing method therefor.
2. Description of the Related Art
As an antenna excited in a higher order mode, one which is disclosed in Japanese Examined Patent Application Publication No. 07-46762 is known. As shown in FIGS. 10 and 11, this antenna has a two-layer structure wherein a microstrip antenna 2 for use in the major mode excitation is placed on a microstrip antenna 1 for use in the higher order mode excitation.
Specifically, in the microstrip antenna 1 for use in the higher order mode excitation, a dielectric substrate 3 having a square shape in a plan view is used, a plan-view circular radiation electrode 4 for use in the higher order mode excitation is formed on the front surface of the substrate, and a ground electrode 5 is provided over the entire back surface of the substrate 3. On the other hand, in the microstrip antenna 2 for use in the major mode excitation, a disk shaped substrate 6 is used, and a radiation electrode 7 for use in the major mode excitation is formed over the entire circular surface of the substrate 6, as well as a center pin 8 is disposed along the center axis of the radiation electrode 4 for use in the higher order mode excitation and the radiation electrode 7 for use in the major mode excitation, thereby ensuring the symmetry between the major mode and the higher order mode.
In the microstrip antenna 2 for use in the major mode excitation, probes F1 and F2 for use in the major mode excitation are disposed at the angular positions of 90xc2x0 with respect to the center pin 8, on the surface of the radiation electrode 7. These probes are provided so as to pass through the substrates 3 and 6 without contacting the radiation electrode 4 for use in the higher order mode excitation and the ground electrode 5.
Also, in the microstrip antenna 1 for use in the higher order mode excitation, probes G10, G11, G20, and G21 for use in the higher order mode excitation are disposed on the 0xc2x0 and 45xc2x0 lines passing through the center pin 8, on the surface of the radiation electrode 4. Specifically, a pair of probes G10 and G11 for use in the first order mode excitation are disposed at the positions symmetrical with each other around the center pin 8 on the line connecting the center pin 8 and the probe F1, and a pair of probes G20 and G21 are disposed at the positions on the 45xc2x0 line which divides the angle formed by the probes F1 and F2 into equal halves. The probes G10, G11, G20, and G21 are provided so as to pass through the substrate 3 without contacting the ground electrode 5.
In the above-described features, when signal powers for the major mode excitation are supplied to the probes F1 and F2 for use in the major mode excitation, with a phase difference of 90xc2x0 provided therebetween using a 90xc2x0 hybrid or the like, a circularly polarized wave is generated. On the other hand, when in-phase signal powers for the higher order mode excitation are each supplied to the probes G10 and G11, and the probes G20 and G21 for use in the higher order mode excitation, and signal powers which have a mutual phase difference of 90xc2x0 are supplied to the probes G10 and G11, and the probes G20 and G21 for use in the higher order mode excitation, a circularly polarized wave in the second order mode (TM21 mode) is generated.
In the microstrip antenna 1 for use in the higher order mode excitation which has the above-described features, since four probes G10, Gil, G20, and G21 for use in the higher order mode excitation are disposed so as to pass through the dielectric substrate 3, the interference (intercoupling) between the radiation electrode 4 for use in the higher order mode excitation and each of the probes G10, G11, G20, and G21 easily occurs, so that there may be a case where the matching between resonant frequencies cannot be achieved.
Also, since the dielectric substrate 3 has a square shape in a plan view, the distances between the periphery of the plan-view circular radiation electrode 4 and the edge line of the substrate 3 are mutually different between the two directions of higher order mode excitation, so that the mutual difference in edge effect, in other words, the mutual difference in the capacitance between the periphery of the radiation electrode 4 and the ground electrode occurs between the two directions. Particularly when the dielectric constant of the substrate 3 is high, this difference becomes significant. The difference in the edge effect would cause a difference in the frequency characteristic of linearly polarized waves between the two directions of the higher order mode excitation. This causes a problem in that circularly polarized waves in a higher order mode reduce the bandwidth in the axial ratio-frequency characteristic.
The present invention has been achieved to solve the above-described problems, and an object thereof is to provide a circularly polarized wave antenna which allows a superior higher order mode excitation to be achieved, and to provide a manufacturing method for the same which allows various electrodes to be easily formed.
In order to achieve the above-described object, the present invention uses the following configurations to solve the above-described problems. The circularly polarized wave antenna in accordance with the present invention comprises a substantially cylindrical substrate comprising a dielectric body; a radiation electrode having a circular shape in a plan view, the radiation electrode being formed on one main surface of the substrate; a ground electrode formed on the other main surface of the substrate; a flat portion formed by flattening a portion of the peripheral side surface of the substrate; and at least two strip shaped feeding electrodes which are formed on the flat portion so as to extend from the ground electrode side to the radiation electrode side.
In the circularly polarized wave antenna with the above-described features, the main surface of the substrate comprises a perfect circle, and the radiation electrode is formed so as to have a diameter smaller than that of the main surface of the substrate so as to be effective diameter to excite the TMn1 (nxe2x89xa72, n: natural number) mode which is a higher order mode. The radiation electrode is disposed coaxially with the main surface of the substrate, and the flat portion provided on the substrate is formed as a flat plane parallel to an imaginary plane (hereinafter, referred to the xe2x80x9caxial planexe2x80x9d) passing the center axis of the substrate.
The two feeding electrodes are disposed so as to form an angle of 90/nxc2x0 (nxe2x89xa72, n: natural number) with respect to the center axis of the substrate, and disposed at the positions which form a plane-symmetry with another axial plane perpendicular to the flat plane. When a signal power is supplied to each of the feeding electrodes, two linearly polarized waves which spatially form 90/nxc2x0, are excited, and by making a phase difference of 90xc2x0 between the two signal powers, a circularly polarized wave in a higher order mode is radiated.
In the circularly polarized wave antenna in accordance with the present invention, it is preferable that the flat portion be provided with a second electrode in conjunction with the feeding electrodes.
In the present invention, since the two feeding electrodes are disposed at angular positions forming 90/nxc2x0 with respect to the center axis of the substrate, the space between the two feeding electrodes remains blank. A second electrode, therefore, is provided making use of the blank between the two feeding electrodes.
The manufacturing method for a circularly polarized wave antenna in accordance with the present invention comprises the steps of forming a radiation electrode having a circular shape in a plan view, on one main surface of a cylindrical substrate, and forming a ground electrode on the other main surface thereof; flattening a portion of the peripheral side surface of the substrate; and collectively forming at least a plurality of feeding electrodes on the flat portion so as to extend from the ground electrode side to the radiation electrode side.
In the manufacturing method for a circularly polarized wave antenna in according with the present invention, since a portion of the peripheral side surface of the substrate is formed into a flat plane, a screen pattern on which electrode patterns are formed, can be placed on the flat plane of the substrate, parallel to the flat plane when printing feeding electrodes using the thick-film screen printing technique. This allows a plurality of feeding electrodes to be collectively formed by printing them at one time.
In addition, in the manufacturing method for a circularly polarized wave antenna in accordance with the present invention, the above-described flat peripheral side surface is formed as a plane parallel to the center axis of the substrate.
In accordance with the present invention, the two main surfaces of the substrate have the same shape, and the width of the flat portion is the same at any position along the center axis direction.
The above and other objects, features, and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.